Process of producing white wood pulp for high grade paper manufacture



i i No Drawing.

Patented Sept. 27, 1932 UNITED STATES PATENT OFFICE GEORGE A. RICHTER,OF BERLIN, NEW HAMPSHIRE, ASSIGNOR 1'0 BROWN COMPANY, OF BERLIN, NEWHAMPSHIRE, A CORPORATION OF MAINE PROCESS OF PRODUCING WHITE WOOD PULPFOR HIGH GRADE PAPER MANUFACTURE I Application filed May 16,

This invention relates to a process of producing an improved wood pulpmore especially from kraft pulp, and possessing not only the importantpapermaki'ng characteristics, including strength and tear resistance, ofthe kraft pulp from which it is derived, but further possessing a purewhite color and far greater freedom from resins and ash content and anotably lower pentosan content than its derivation material. The productproduced by the process of the present invention is thus one whichfulfills the desiderata in two fields of papermaking, namely, thatdevoted to the manufacture of bond, ledger, and writing papers, whereinwhiteness of fiber is a prime desideratum, and that devoted to themanufacture of envelope, bag papers, and the like, wherein strength andtear resistance, as well as whiteness of fiber, are highly prized.

Those skilled in the art well know that the conversion of kraft pulp towhiteness is a difficult task, which when accomplished by the use of ahypochlorite bleach liquor is accompanied both by exceedingly highbleach usage and marked injury to the papermaking characteristics of thefiber. The process of the present invention, however, is aimed at thepreservation of the valuable papermaking characteristics of kraft pulpwhile, re-

I moving from such pulp so much of its coloring matter and otherconstituents as are undesirable from a papermakers standpoint. Theprocess of the presentinvention does not result in a product which is ofextraordinarily high alpha cellulose content, for if it be attempted torefine kraft pulp markedly'in so far as'concerns increasing its alphacellulose content, it loses especially in-such properties as strength,even though it may be improved in other respects, such as softness andstability.

In practising the process of the present invention, I proceed asfollows. A raw kraft pulp is subjected to the action of. chlorine wateror to hypochlorite bleach, liquor, or to a succession of such reagents,without visible whitening of the pulp. In fact, the chlorine and/orhypochlorite should be used in such amount that the pulp after treatmenthasa 1931. Serial No. 537,997.

distinctly different hue than the raw kraft I pulp. For example, rawpulp may be treated at temperatures of from 50 to- 100 F. for two to sixhours, as a 4% to 6% stock suspensionin chlorine water containing 4% tov8% chlorine, based on pulp. During this treatment, the pulp assumes adeep orange color. The treated pulp is then washed free of treatingreagent and reaction products. When the washed pulp is tested, itexhibits a lower lignin content than that present in the original pulp.Ligneous matter is thus removed from the pulp without injury to itspapermaking characteristics. The prec'hlorinated stock may beantichlored or neutralized, if

desired, to remove residual chlorine before it 1s subjected to a washingoperation.

The washed pulp is then treated with an alkaline liquor preferablysimilar to the cooking liquor used in the kraft process of fiberliberation, namely, one containing both caustic soda and sodiumsulphide. Preferably,'the liquor is one containing 2% to4% alkali,calculated as caustic soda equivalent, L

but 40% of which alkali'is present in the form of sodium sulphide. Thepulp is suspended in this specific liquor as an 8% to10% stocksuspension at about 40 to 70 (1., for from about one to four hours.During this treat- .ment, the pulp isfurther freed from'ligneous matter,but Without impairing its paper marking characteristics. The pulp isthen washed, at which stage it has alignin content of from 1% to 2.5% asagainst a lignin content of from 3% to thus seen that the process thusfar is directed largely at the selective removal from the pulp ofligneous matter, the initial treatment selectively removing some of thelignin and chlorinating the rest, and-the second treat ment beingconducted in an alkaline solution under conditions to dissolve out thechlori- 7% in the raw kraft pulp. It is ing'particular amounts of bothcaustic soda.

and sodium sulphide. Such temperature conditions are conduciveto'thedesired dissolving out action without'the markedrefinsolution ofalkali hereinbet'ore described, say

- 90 to 100 C. or higher, it is found that there is a marked sacrificeinthe tear resistance of the pulp, even though its Mullen or popstrength is not appreciably afi'ected. On the other hand, if thetreatment of the pulp be carried out at temperatures lower than thosespecified with the solution of alkali hereinbefore described, there is aless effective removal of lignin from the pulp and a strong tendency toreduce the Mullen and tensile strength of the pulp; and there is apt tobe an increase in the alpha cellulose content of the pulp to a pointwhere it approaches the softness and other characteristics of a refinedwood pulp. It is thus seen that the treatment of the pulp with asolution of alkali to produce the desired product is conducted, inaccordance with the present invention, within a tempera ture andalkalinity range designed to effect an extensive elimination of ligninfrom the pulp without appreciably affecting its alpha cellulose contentor its strength and tear resistance characteristics. In this connection,it is to be observed that alkaline liquors other than that-mentioned maybe used so long as the conditions of alkalinity and temperature of theliquor are such as will remove largely residual ligneous reactionproducts from the .35 um hypochlorit'e bleach, say 2% to 3%, based pulp,as a re ult of the initial treatment of the pulp with oxidant, whilesubstantially preserving the alpha cellulose content of the pulp. Forinstance, another alkali, such as sodium carbonate, may be used in lieuof caustic soda in an alkaline liquor of about 4% alkali content,calculated as caustic soda equivalent, about 40% of which alkali may bepresent in the form of sodium sulphide. The temperature of digestion insuch case should be about C. to eifect the desired removal of ligneousmatter in a reasonably short period of time. Such an alkalineli uor,containing sodium carbonate and of a out 40% sulphidity, is'available ina kraft mill, as a result of the recovery operations practised therein.Thus,-the smelted inorganic residuum issuing from the smelting furnaceof a kraft mill consists essentially only of sodium carbonate and sodiumsulphide, the sodium sulphide content-of the smelt ranging from Thesmelt may be dissolved in water, and, after filtration, may be adjustedin alkali con tent for use in treating kraft pulp which has .undergonean initial treatment with oxidant as hereinbefore described. If desired,the al kaline liquor may be a straight sodium car-v bonate solution ofthe strength hereinbefore indicatedcontaining a small amount ofsodisubsequent bleaching of the pulp to whiteness.

It may further be mentioned that the alkaline treatment may be effectedin two stages, first with comparatively dilute solution of such alkaliesas lime or sodium carbonate at v elevated temperatures, and then withthe stronger and more expensive alkaline solutions at lower temperaturesas has been described in detail, the initial alkaline treatment servingto eliminate from the pulp essentially only the easily removablechlorination reaction products. 4

The alkaline-digested and washed pulp is now ready to be bleached.Bleaching, too, must be carried out in a special way, in order to arriveat my novel product. To this end, the pulp is preferably subjected to aseries of three independent bleaching treatments. The first twobleaching treatments consist in maintaining the pulp for three to sixhours as a 5% suspension in hypochlorite bleach liquors at 7 0 F.,.about3% calcium hypohypochlorite based on pulp being used in the firsttreatment, about 2% calcium hypochlorite based on pulp being used in thesecond treatment, and about 0.5% caustic soda based on pulp being addedto the bleaching liquors in each treatment. The pulp is washed andthickened between the first two treatments, and again washed andthickened after the second treatment. The third and final bleachingtreatment may consist in treating the pulp with sufiicient bleach,rendered alkaline throughout the bleaching operation by the presence offree alkali, to produce a product of pure whiteness, but other bleachingagents, such as peroxides, permanganates, and perborates may be used inthis final bleaching treatment. At the end of this treatment, the pulpis washed,.but when permanganates are used, it has a light tan colorbecause of residual manganese oxides clinging to the fiber. The oxidesmay be completely removed by treating the pulp with a dilute sulphurousacid or bisulphite solution and washing.

The resulting product, which is in pure- 3 deed, the succession'ofbleaching treatments I and alternate washings preserve the physicalcharacteristicsof the pulp while converting it to a white condition; andthe final bleaching treatment is conducted whenthe pulp is nearly purewhite, and so requires such a dilute solution of bleaching agent as toacquire pure whiteness with no observable physical degradation.

, The following table indicates the transformation efiected upon thegiven kraft pulp in practising the process of the present inven-.

tion:

tlilroduct of e process. of present invention Color Brown. Pure white.Total cellulose (per cent). 93. 0 98. 5 99. 5 Alpha. cellulose (percent) 89 p 87 89 Cu. No 1.2 1.0- 2.0 Ash (per cent) .6- 1.0 0.1- 0.3Resins (per cent) 2- .4 O. 1- 0.3 Pentosans (per cent 7 9 3.0- 5. 0Strength 150 -160 150 160 Tear resistance 250 270 250 270 It is to beobserved that the product produced by the process of the presentinvention, in addition to having the papermaking characteristics,including strength and tear resistance, of kraft pulp, is of pure-whitecolor and has an extremely low resin contcnt,in fact being lower inresinous specks than most bleached sulphite pulps on the market. It hasa much higher total cellulose content than the original pulp, butitsalpha cellulose content is practically the same as that of theoriginal pulp. Its pentosan content is somewhat lower than that of theoriginal pulp, but there are sufficient pentosans therein in a formimparting toughness or strength to the sheet, Without developing thebrittleness imparted to a kraft pulp when it is whitened, for example,by successive treatments with sulphurous acid and hypochlorite bleachliquor.

While the product has been described'as being produced more especiallyfrom kraft pulp, it may be prepared by my process from other suitablealkaline-cooked wood pulps, for instance, the long-fibered soda pulpsobtained by efi'ecting fiber liberation from coniferous woods inso-called soda cooking liquors.

I do not herein claim the novel product resulting from the process ofthe present invenion, as such a product has been'described and claimedin my application Ser. No.

496,566, filed November 18, 1930, now Patent No. 1,809,287 dated June-9, 1931. So far as subject matter is concerned, this application-isa continuation in part of my application Serial No. 189,461, filed May6, 1927 and Serial No. 292,325, filed July 12, 1928.

VVhat'I claim is:

1. A process which comprises treating unbleached, alkaline-cookedwoodpulp with an oxidant of the character of chlorine or hypo chlorite toreact upon ligneous groups in such pulp, digesting the pulp at about 40to C. in an alkaline liquor-containing about 2% to 4% alkali, calculatedas caustic soda equivalent, to remove largely ligneous reaction productsfrom such pulp while substantially preserving unchanged the alphableached, alkaline-cooked wood pulp with an oxidant of the character ofchlorine or hypochlorite to react upon ligneous groups in such pulp,digesting the pulp at about 40 to 70 C. in an alkaline liquorcontainingabout 2% to 4% alkali, calculated V as caustic soda equivalentbut in the form of;caustic soda and a sodium sulphide, toremove largelyligneous reaction products from such pulp while substantially preservingunchanged the alpha cellulose content of such pulp, and bleaching thedigested pulp to whiteness while substantially preserving its physicalcharacteristics.

3. A process which comprises treating unbleached, alkaline-cooked woodpulp with an oxidant of the character of chlorine or hypochlorite toreact upon ligneous groups in such pulp, digesting the pulp at about 40to 70 C. in an alkaline liquor containing about2% to 4% alkali,calculated as caustic soda equivalent but in the form of'sodiu'mcarbonate and sodium sulphide, to remove largely ligneous reactionproducts from such pulp while substantially preserving unchanged thealpha cellulose content of such pulp, and bleaching the digested pulp towhiteness while substantially preserving its physical characteristics.

' 4. A process which comprises treating unbleached, alkaline-cooked woodpulp with chlorine Water containing 4% to 8% chlorine, based on pulp,until the chlorine is substantially consumed, digesting the chlorinatedpulp for about one to four hours at about 40 to 70 C. in an alkalineliquor containing about 2% to 4% alkali, calculated as caustic soda.equivalent, toremove largely ligneous reaction products from such pulpwhile substantially preserving unchanged the alpha cellulose content ofsuch pulp, and bleaching the digested pulp t0 whiteness whilesubstantially preserving its physical characteristics.

5. A process which comprises treating unbleached, alkaline-cooked woodpulp with an oxidant of the character of chlorine or hypochlorite toreact "upon ligneous groups in such pulp, digesting the pulp in analkaline liquor under conditions of alkalinity and temperature of liquorcorrelated with time of digestion to remove largely ligneous reactionproducts from such pulp while'sub'stantially preserving unchanged thealpha cellulose content of such pulp, and subjecting the digested 6. Aprocess which comprises treating unbleached, alkaline-cooked wood pulpwith an oxidant of the character of chlorine or hypochlorite to reactupon ligneous groups in such 5 pulp, digesting the pulp in an alkalineliquor under conditions of alkalinity and temperature of liquorcorrelated with time of digestion to remove largely ligneous reactionproducts from such pulp while substantially pre- 1 servinjg unchangedthe alpha cellulose content 0 such pulp, and subjecting the digestedpulp to more than one distinctly alkaline hypochlorite bleachingtreatment and alternate washings under conditions to preserve thephysical characteristics of such pulp substantially unimpaired.

In testimony whereof I have afiixed my signature.

\ GEORGE A. RICHTER.

